Observation of cold, long-lived antiprotonic helium ions

Cold, two-body antiprotonic helium ions p /sup 4/He/sup 2+/ and p /sup 3/He/sup 2+/ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers n/sub i/=28-32 and l/sub i/=n/sub i /-1 have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature T~10 K by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of p /sup 3/He/sup 2+/ against annihilation induced by collisions were shorter than those of p /sup 4/He/sup 2+/, and decreased for larger-n/sub i/ states.